Mass spectrometers comprising a gas chromatograph coupled to an Electron Ionisation (“EI”) or Chemical Ionisation (“CI”) ion source are well known. A gas chromatograph comprises a packed column or open capillary tube located in a heated chamber. Analyte gas molecules are caused to pass through the column. Gas molecules having different sizes and structures will take different amounts of time to elute from the gas chromatograph.
Ions which emerge from the gas chromatograph are then commonly ionised either by an Electron Ionisation ion source or by a Chemical Ionisation ion source.
An EI ion source comprises an ion chamber through which an electron beam is passed. Analyte gas molecules interact with the electron beam and are subsequently ionised. The ionisation process is commonly referred to as being a hard ionisation process in that the analyte molecules are caused to fragment as a result of the ionisation process. The resulting EI fragment ions are then mass analysed.
A CI ion source utilises a reagent gas (e.g. methane or ammonia) and may be operated in either a positive or negative mode of operation. Neutral reagent gas is arranged to be ionised by interactions with free electrons emitted from a filament. The resulting reagent ions are then caused to interact and ionise neutral analyte molecules resulting in the formation of analyte ions. The resulting analyte ions are then mass analysed.
The coupling of a gas chromatography column with an EI or CI ion source and a mass spectrometer is a powerful technique that is widely used in many laboratories.
Conventionally, EI and CI ion sources comprise ion source chambers made from stainless steel. Stainless steel is considered to be relatively inert and non-reactive. However, conventional EI and CI ion source chambers need regular cleaning in order to maintain high performance.
Conventional EI and CI ion source chambers can suffer from increased surface contamination following regular analysis of complex matrix extracts such as urine, saliva, plasma, whole blood, waters and soils.